To prevent or mitigate an AIDS epidemic among injecting drug users (IDUs), effective activities need to be implemented on a large enough scale to reach and assist sufficient numbers of drug users and thereby change their risk behaviors related to drug use and sex. Recent work by UNAIDS on "high coverage sites", adopting the above strategies, has shown that one of the key elements in achieving high coverage is ongoing and sophisticated advocacy. High coverage harm reduction sites were studied through literature search and site visits, including key informant interviews, review of service statistics, and data analysis, in order to document the steps that led to scaling up, the way coverage was defined in these sites, and the lessons learned from their efforts. Syringe-exchange programs can achieve high coverage of IDUs. Monitoring to determine regular reach (those who are in regular contact with harm reduction services) should be added to uniform data collection carried out by harm reduction programs. Advocacy is crucial to achieving high coverage.